The method relates to an optical device for determining the relative position of two vehicles, one of which carries an active installation and is referred to below as the "chaser" while the other one is referred to below as the "target" and carries markers that are passive during such determination. A particularly important, but non-exclusive application of the invention lies in the field of measuring the distance, the position, and/or the orientation of the target relative to the chaser, in particular for enabling two spacecraft to rendezvous and dock.
A device is already known (EP-A-0 254 634) that enables the relative attitude and distance between a chaser and a target to be determined, said device comprising, on the chaser, a camera for forming an image of a field of view having determined angular extent, and on the target, a set of several markers spaced apart in such a manner that they are all contained in the angular field when the distance between the chaser and the target is large and the field of the camera is properly centered relative to the target.
Such a measurement device in which the markers are generally constituted by reflectors distributed in a determined geometrical pattern and illuminated by a source of light pulses carried by the chaser, can be used for determining the distance between the chaser and the target by measuring the go-and-return time of the light, and it enables the attitude of the target relative to the chaser to be determined at relatively short range by comparing the image obtained with the known pattern in which the markers are distributed.
It becomes extremely difficult to measure relative attitude and misalignment when the distance between the chaser and the target is less than a threshold. There are two reasons for this. Firstly, the focusing of the image on the detector (which is generally constituted by a camera having a matrix of sensors) is degraded when the distance between the chaser and the target becomes small. In addition, below the threshold, the angular separation between the markers tends to exceed the angular field of the detector.
By way of example, FIG. 1 shows the field of view of a detector that can be considered as being representative, the field extending over an angle of 10.degree. in one direction and 13.3.degree. in the other direction. A pattern constituted by reflectors disposed at the corners and at the center of a square-based pyramid having a base of side 10 cm appears in the field in the manner shown in FIG. 1 when the range is 1 meter. At a range of less than 0.3 m, it is no longer possible to keep all of the markers in the field, even when there is no aiming error.
Another problem is constituted by the impossibility of giving the camera an optical system that retains focusing from infinity down to very short range. In practise, the loss of focus when the range between the detector and the target drops below 50 cm, causes the image provided by the camera to have a spot whose diameter is large in number of pixels. For a camera having a matrix made up of charge coupled devices (CCD) of the type conventionally used, and using the parameters mentioned above, the out-of-focus spot has a diameter of 9 pixels at a range of 50 cm.
The idea that comes to mind for avoiding the above problems consists in providing a plurality of patterns on the target each constituted by retroreflectors of a diameter that is optimized to take account of diffraction and of the angular field that corresponds to a single pixel. For example, this leads to a plurality of patterns being provided having the following unit diameters:
50 cm for ranges of 100 m to 1 km; PA0 12 cm for ranges of 10 m to 200 m; PA0 1.2 cm for ranges of 2 m to 20 m; and PA0 5 mm for ranges of 0.5 m to 4 m.
Nonetheless, that technique does nothing to solve the problem of loss of focus which makes useful measurement impossible, in practice, at ranges of less than about 50 cm. Calculating the center of gravity of a spot caused by loss of focus becomes very difficult.